The present invention relates to electroless nickel plating solutions and in particular to stabilizers for electroless nickel plating solutions and methods of use thereof.
Electroless metal deposition solutions (xe2x80x9celectroless plating solutionsxe2x80x9d) deposit metal over a catalytically active surface by chemical reduction in the absence of an external electric circuit. Those solutions containing nickel are used in a wide range of industrial applications, particularly in the manufacture of electronic appliances and other articles. Nickel-boron and nickel-phosphorus coatings are recognized in the art for their hardness and associated wear-resistance. See, for example, U.S. Pat. Nos. 2,726,170; 3,045,334; 3,378,400; 3,738,849; 3,674,447; 3,432,338; and 5,019,163. Electroless nickel plating solutions are also used in the manufacture of electronic printed circuit boards, such solutions having been described, for example, in U.S. Pat. Nos. 2,690,401; 2,609,402; 2,762,723; 3,420,680; 3,515,564; and 4,467,067, all of which are incorporated herein by reference.
Typical electroless nickel plating solutions generally comprise a water-soluble nickel salt, a water-soluble alloying salt (if an alloy is present), a reducing agent, and a chelating or complexing agent. Additives may also be added in relatively low concentrations to enhance various characteristics of the solution of plated article. One continuing need with respect to electroless nickel plating solutions has been the need to increase the stability of the solution. It has been found, however, that in certain instances the addition of stabilizers to meet this need interferes with the formation of the nickel coating, in that during the formation of the nickel coating the accelerator co-deposits in the nickel. Some stabilizers are also known to cause discoloration of the deposit. For example, lead or cadmium can act as a stabilizer, but their addition can make the deposits shiny). Moreover, it is well known that stabilizers may cause step- or skip-plating. They can even prevent the initiation of the deposition completely if their concentration is too high. There accordingly remains a continuing need in the art for effective stabilizers for electroless nickel plating solutions that do not adversely affect the deposition process.
An electroless nickel plating composition comprises nickel, a reducing agent, a complexing agent, and an accelerator, wherein the accelerator is a mesoionic compound in an amount sufficient to accelerate the rate of deposition of the composition. The mesoionic compound may comprise sulfur. Most preferably, the mesoionic compound is a triazolium compound having the structure (I): 
wherein
R1 is a substituted or unsubstituted alkyl, alkenyl, thioalkoxy, or alkoxycarbonyl group having from 1 to 28 carbon atoms; a substituted or unsubstituted cycloalkyl group having from 3 to 28 carbon atoms; a substituted or unsubstituted aryl group having from 6 to 33 carbon atoms; a substituted or unsubstituted heterocyclic ring having from 1 to 28 carbon atoms and one or more hetero atoms, such as N, O, and/or S; an alkyl, cycloalkyl, alkenyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted aromatic ring; or an alkyl, cycloalkyl, alkenyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted heterocyclic ring having 1 to 28 carbon atoms and one or more heteroatoms such as N, O, and/or S;
R2 is a substituted or unsubstituted amine group having from 0 to 25 carbon atoms; a substituted or unsubstituted alkyl, alkenyl, or alkoxy group having from 1 to 28 carbon atoms; a substituted or unsubstituted cycloalkyl group from 3 to 28 carbon atoms; a substituted or unsubstituted acyloxy group having from 2 to 25 carbon atoms; a substituted or unsubstituted aryl group having from 6 to 33 carbon atoms; a substituted or unsubstituted heterocyclic ring having from 1 to 28 carbon atoms and one or more hetero atoms, such as N, O, and/or S; an alkyl, cycloalkyl, alkenyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted aromatic ring; or an alkyl, cycloalkyl, alkenyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted heterocyclic ring having 1 to 25 carbon atoms and one or more hetero atoms such as N, O, and/or S);
R3 is a substituted or unsubstituted amine group having from 0 to 25 carbon atoms; a substituted or unsubstituted alkyl, alkoxy, or alkenyl group having from 1 to 28 carbon atoms; a substituted or unsubstituted cycloalkyl group having from 3 to 28 carbon atoms; a substituted or unsubstituted acyloxy group having from 2 to 25 carbon atoms; a substituted or unsubstituted aryl group having from 6 to 33 carbon atoms; a substituted or unsubstituted heterocyclic ring having from 1 to 28 carbon atoms and one or more hetero atoms, such as N, O, and/or S; an alkyl, cycloalkyl, alkenyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted aromatic ring; or an alkyl, cycloalkyl, alkoxyalkyl, aryl, or phenoxy group connecting to a substituted or unsubstituted heterocyclic ring having 1 to 25 carbon atoms and containing one or more hetero atoms such as N, O, and/or S; said R1, R2, and R3 may further combine with each other to form a 5-, 6-, or 7-membered ring; and
X is a moiety covalently bound to the ring and capable of being negatively charged, for example a chalcongen such as a sulfur atom or an oxygen atom.
Alternatively, the mesoionic compound is a tetrazolium compound having the structure (II) 
wherein R1, R2, and X are as defined above.
In another embodiment, a method for the manufacture of a coated article comprises contacting the surface of the article with the above-described coating composition, preferably at pH about 3 to about 11 and at an elevated temperature of about 40 to about 95xc2x0 C. The method is particularly useful for coating the surface of an article of manufacture that is subject to exposure to corrosive conditions or one subject to sliding or rubbing contact with another surface under unusual wearing and bearing pressures.
Another embodiment comprises an article of manufacture having a surface and a coating deposited thereon, wherein the coating comprises about 95 to about 99.9 weight percent nickel and about 5 to about 0.1 weight percent boron or from about 85 to about 99 weight percent of nickel and about 15 to about 1 weight percent phosphorus.